This invention relates to a drainage system, such as a drainage system for a roof, and an apparatus for treating runoff, such as rainwater runoff from a roof.
Drainage systems for roofs are well known. Typically, a gutter is provided at an edge of the roof for collecting rain which has fallen onto the roof. The gutter is connected to a downpipe which extends downwardly from the gutter.
When it rains, water which has fallen on the roof runs off the roof and collects in the gutter. The collected water flows along the gutter and drains through the downpipe. During a heavy downpour, the level of water collected within the gutter increases. As the level of water increases, the flow rate exceeds the flow capacity of the downpipe. Inlet flow through the downpipe inlet therefore undergoes a transition from a partial flow condition in which air is drawn in through the downpipe inlet and entrained within the water along the downpipe to a full flow condition in which no air is drawn through the downpipe inlet.
During this transition, air trapped within the downpipe as the flow rate increases is forced back through the pipe inlet, which causes glugging. Glugging causes the downpipe and associated structure to vibrate: a phenomenon often referred to as water hammer. The vibration is undesirable since it causes noise and can damage the drainage system or cause it to detach from the roof/building to which it is secured.
A further problem associated with existing drainage systems is that the long drop provided by the downpipe causes water to exit the downpipe at a high velocity. The high velocity water can damage downstream components, such as drainage pipes of treatment vessels. Consequently, the length, diameter and/or orientation of the downpipe is often constrained by the need to maintain an acceptable flow rate that will not damage components situated at the outlet of the downpipe.